veth: refine napi usage

After the previous patch, when enabling GRO, locally generated
TCP traffic experiences some measurable overhead, as it traverses
the GRO engine without any chance of aggregation.

This change refine the NAPI receive path admission test, to avoid
unnecessary GRO overhead in most scenarios, when GRO is enabled
on a veth peer.

Only skbs that are eligible for aggregation enter the GRO layer,
the others will go through the traditional receive path.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>